A direct conversion radiation detector, as used in, for instance, computed tomography (CT) imaging, detects radiation by directly converting incoming radiation into electrical signals, which may, amongst others, be used for photon counting. Direct conversion detectors, and particularly those with a Cadmium Zinc Telluride (CdZnTe or CZT) direct conversion layer, are sensitive to moisture. Over time, exposure to moisture may change properties of a photon-electron conversion layer and a surface of the direct conversion detector. This may result in an increase of leakage current, which negatively influences photon counting properties of the direct conversion detector. Also other direct conversion materials, e.g. Cadmium Telluride or (other) single crystal materials, are similarly influenced by moisture.
In a shorter time frame moisture may cause a short circuit between a cathode and an anode via a direct conversion layer surface, which almost certainly will destroy the detector's integrated circuit (ASIC) and sensor. Furthermore, moisture in the direct conversion layer in combination with high used voltages in the device may cause an increased chance of breakdown.
And, moisture sensitivity in the radiation detector is not exclusive to the direct conversion layer. Also properties of other components, such as electrodes and connects, may be adversely affected by moisture. For the same reason the problem also extends to non-direct conversion radiation detectors. On top of that, non-direct conversion radiation detectors with scintillator materials, such as Cesium Iodide, may likewise have similar moisture sensitivity and the related problems associated with this.
To prevent moisture degradation, encapsulation is applied to the direct conversion detector. Encapsulation not only reduces moisture degradation, it may also improve scratch resistance and chemical resistance of the detector and its components. Encapsulation may be achieved by providing a liquid encapsulation layer to the detector, for instance by applying an epoxy-based material. Such liquid encapsulation materials are commonly applied to a detector (CZT) tile with electrodes and afterwards openings are formed on detector pixels for contacting by photolithography. Application of the encapsulation is troublesome as good coverage of the corners and edges is difficult to obtain when using a liquid based material.
As an alternative other encapsulation materials, including SiO2, AlN, SiN and Al2O3 are known that are not applied as a liquid, but using physical vapor deposition (‘sputtering’) or chemical vapor deposition (CVD). These processes cause the substrate, including the direct conversion material, to be heated up. With sputtering the substrate may heat up to 100 to 150 degrees Celsius. For CVD the substrate is exposed to 200 to 700 degrees Celsius. These processing temperatures are too high for use with the direct conversion material, because this will negatively influence the properties of the direct conversion material (such as single crystal properties of CZT). The temperature should not exceed 100 degrees Celsius, preferably not 80 degrees Celsius for extended periods of time. It is possible to perform CVD at a lower temperature, e.g. at 100 degrees Celsius, but this results in an insufficient moisture barrier, because pinholes, insufficient layer build-up and other defects are much more prevalently present. Said maximum processing temperature is especially important for detector tiles of those used in CT detectors, since any (material) defects may influence the fast sensor operation required for CT.
In U.S. Pat. No. 7,700,923 B2, and in other publications, parylene, an organic polymer, is disclosed as an encapsulation material, which is deposited from the gas phase at room temperature to form a layer with good humidity barrier properties.
However, a problem with parylene, and the other encapsulant materials, is that after encapsulation a lithographical step is required to locally open the encapsulation material to allow for connecting detector pixels to the ASIC. The encapsulation is therefore not fully conformal, somewhat limiting the moisture barrier, as well as chemical and mechanical resistance.